1. Field of the Invention
The invention relates to a method of improving the restoration of images supplied by a side looking sonar and in particular a sonar comprising a digital acquisition chain.
2. Description of the Prior Art
Lateral aimed sonars are, it will be recalled, sonars having acoustic transmitting-receiving transducers fixed to the hull of a ship or better still to the hull of a streamlined body or "fish" towed immersed by the ship, and whose main transmitting-reception axes are orientated in oblique directions in a transverse plane perpendicular to the path of a ship.
For transmission-reception a plurality of transducers are aligned in a direction parallel to the longitudinal axis of the ship or fish, so that their directional patterns have a small opening angle on each side of the vertical transverse plane and a wider opening angle in this same plane.
The transmitting transducers are fed at regular intervals and send acoustic pulses in an oblique direction towards the bottom of the water. These pulses irradiate or insonifie sea bed on one side or on both sides of the ship or fish over a narrow strip of elongate shape perpendicular to the direction of travel and are backscattered to the surface.
The receiving transducer or transducers, because of their directional pattern, are adapted for picking up preferably the echoes coming from the irradiated strips.
The signals received by the transducers, echoes of the transmitted pulses, are amplified preferably with an amplification gain increasing with time for correcting the amplitude variations due to the obliqueness of the propagation paths, then demodulated so as to obtain low frequency signals representative of the envelope of the signals received.
The demodulated signals are recorded and obliqueness corrections, taking into account the water height under the boat, are made to the recordings so as to obtain a linear representation of the sea bed.
The images which are obtained of the surface of the sea bed are in general unsatisfactory in so far as the zones corresponding to a small obliqueness of the acoustic paths are concerned. That is due in particular, as will be seen in greater detail in the description, to the fact that the transverse dimensions of the surface portions reflecting the echos received during the same time interval are much larger at the beginning of each reception phase, i.e. for the directions close to the vertical than for the directions more slanted with respect thereto, and consequently to the low transverse resolution power for the near zones.
In sonars comprising a digital acquisition chain, this time interval is for example the period (or pitch) at which the demodulated signals are sampled.
The method of the invention overcomes the above mentioned drawbacks.